In recent years, shorter wavelength laser sources used as light sources for reproducing or recording information for an optical disc which is an optical information recording medium have been developed. For example, laser sources having wavelengths of from 380 nm to 420 nm including blue violet semiconductor laser diodes and SHG lasers which convert the wavelength of infrared semiconductor laser by utilizing second harmonic generation, have been developed.
By using these blue violet laser sources, it becomes possible to record information of 15 GB-20 GB onto an optical disc having diameter of 12 cm, when employing an objective lens having the same numerical aperture (NA) used for DVD (Digital Versatile Disc). It becomes possible to record 23 GB-25 GB information onto an optical disc having diameter of 12 cm, when raising NA of an objective lens to 0.85. An optical disc and an optical magnetic disc both employing a blue violet laser source will be generically named “a high density optical disc” in this specification.
As the high density optical disc, so far, two types of optical disc formats have been proposed. One is an optical disc format using an objective lens having NA 0.85, the thickness of a protective layer is 0.1 mm, which is called Blu-ray disc, (hereinafter, which will be called BD). The other is an optical disc format using an objective lens having NA 0.65-0.67, the thickness of a protective layer is 0.6 mm, which is called HD DVD, (hereinafter, which will be called HD). Currently, DVDs and CDs (Compact Discs), onto which various kinds of information are recorded, are sold in the market place.
When, for example, assuming that an optical pickup apparatus is configured into a note type personal computer, the optical pickup apparatus needs to be a compact size. When trying to minimize the size of the optical pickup apparatus, making the effective diameter of an objective lens small is effective. However, in general, when suppressing the effective diameter of the objective lens, the focal length of the objective lens becomes short. As a result, there exists a problem that a working distance of the objective lens, which is necessary to avoid contacts of an optical disc with the objective lens, becomes short. Particularly when trying to establish compatibility of the objective lens between BD and CD or between BD and DVD, it is necessary to secure an enough working distance of the objective lens when using CD or DVD. Accordingly, when using BD, a further longer working distance becomes necessary. Further, in order to decrease the load of an actuator for driving an objective lens, a light-weight objective lens is required.
In order to solve the problem described above, Japanese Patent Applications Open to Public Inspection (JP-A) Nos. 2003-5032, 2003-114380, 2003-107341 and 2003-91854 disclose several kinds of objective lenses.
Since the objective lens disclosed in Example 9 in JP-A No. 2003-5032 has a small diameter, it helps optical pickup apparatus to be made small. However, there is a problem that due to the high refractive index, the weight of the lens becomes heavy and the material cost of the lens goes up. Each of the objective lenses disclosed in Example 1 in JP-A No. 2003-107341 and Examples 1, 2, and 3 in JP-A No. 2003-91854 has a small diameter and a low refractive index. However, since these examples have an excessively smaller diameter, these have problems that their working distances become too short. Especially, they have large difficulty about obtaining working distance required as a compatible objective lens commonly used for information recording or reproducing on optical discs in a plurality of kinds having different formats. Further, each of objective lenses disclosed in Example 9 in JP-A No. 2003-5032, Example 2 in JP-A No. 2003-114380, Example 1 in JP-A No. 2003-107341, and Examples 3, 4, 5, and 6 in JP-A No. 2003-91854 has a problem that a suitable working distance on actual use is hard to be secured with an optical property required on actual use being kept since a thickness on the optical axis and focal length for a refractive index of the each objective lens is not optimized with good balance.